The use of pressurized gas in plastic injection molding to make hollow plastic parts is now widely used. One problem with the use of gas is that sometimes the gas flow path cannot be completely controlled and the gas will flow from the runner in the mold back through "drops" in the mold and then back into a plastic injection nozzle to contaminate the plastic in the nozzle. Gas trapped in the nozzle area tends to foam the plastic resin in that area during venting. This foamed plastic causes blemishes (i.e. splay) in the next part when injected into the mold.
U.S. Pat. No. 4,943,407 discloses both a stationary disk-shaped insert and a movable pin disposed within a sprue body to prevent such contamination.
U.S. Pat. No. 4,942,006 discloses a method and apparatus for gas-assisted injection molding in which variable pressure is applied to a piston of a valve for a nozzle causing the tip to open and close preventing a gas from entering the flow path.
U.S. Pat. No. 5,149,546 discloses a check valve for use in gas-assisted injection molding having a tapered member that seats into a corresponding tapered valve chamber for preventing reverse flow.
U.S. Pat. No. 5,080,570 discloses gas-assisted injection molding having a reciprocating shut-off pin in the non-viscous fluid flow passage of the shut-off needle for controlling fluid flow.
U.S. Pat. No. 5,232,711 discloses a method and system for gas-assisted injection molding including a sealed pin or valve having an orifice that is movable between opened and closed positions.
U.S. Pat. No. 5,507,637 discloses a hot runner sliding nozzle-manifold assembly employing a clamping ring to apply a clamping force to the nozzle housing for preventing leakage of molten plastic.
U.S. Pat. No. 4,740,150 discloses a gas-assisted injection molded apparatus having a mold valve for venting gas pressure and U.S. Pat. Nos. 5,582,851 and 4,682,943 disclose multi-plate injection molding per se.